Parasitism and ImmiDiit i 



47 



which are fixed, and breed true, and thus two generations are sufficient to pro- 

 duce and fix the new variety. There is also a definite proportion between the 

 fixed and unfixed sorts. In the particular instance given, where two con- 

 trasted characters are concerned, the susceptibility to rust is called the domi- 

 nant character, because it prevails over the other or recessive character in the 

 first generation, and this holds good irrespective of which plant was pollen- 

 bearing or seed-producing. In the second generation, the dominant " rusty " 

 plants and the recessive " immune " plants appear in the ratio of ."] to 1, as 

 nearly as possible. 



Smut Immunity. 



Although I am not at present (1909) in a position to give such defiiiite 

 results for smut as have just been given for rust, yet experiments are under 

 way to settle the point as regards Stinking Smut, or Bunt. The difficulty 

 here is to secure wheat-plants which are immune to this smut when 

 grown under conditions favorable to its development, such as the seed- 

 wheat being dusted with the spores, but this difficulty is being gradually 

 overcome. 



There is a variety grown here, known as AUora Spring, which is most suscep- 

 tible to bunt, having yielded 95.5 per cent, of bunty plants when the seed 

 was coated with spores, while under the same conditions, Minnesota Blue Stem, 

 a strong flour variety, was the least susceptible of ten varieties tested, 

 only producing 12 per cent. But while a small proportion of rust is admissible 

 without seriously interfering with the yield or the quality of the grain, a very 

 small percentage of Stinking Smut is objectionable, and it is necessary, if treat- 

 ment of the seed is to be dispensed with, to have a variety or strain which is 

 absolutely free. Experiments in the direction of producing bunt-resisting 

 wheats have been hitherto mainly carried out by Farrer and Pye. The work 

 of Farrer is being continued by Sutton at the Cowra Experiment Farm. 



Experiments of Farrer and Sutton in New South Wales. 



When the grains of dift'erent varieties of wheat are thoroughly dusted with 

 bunt spores and sown at the same time alongside each other, it is found that 

 the percentage of bunty plants varies considerably according to the variety, 

 basing the calculation upon the number of plants which grew in each case and not 

 upon the number sown. Thus, in experiments carried out by Farrer^ with ten 

 different varieties, the percentage varied from 95.5 to 12, and the proportion 

 which remained clean might be owing to the fact that either the plants them- 

 selves, or the seeds from which they grew, possessed the property of resisting 

 infection. But there are various other causes which might produce this re- 

 sult, even on the assumption that the bunt spores germinated, such as germ- 

 tubes which penetrated the seedlings failing to reach the growing point, or the 

 growth being too rapid for the mycelium to keep up with it. So that the only 

 way to settle the point was to harvest the clean plants separately and sow the 

 seed after inoculation, in order to see whether any of them transmitted the 

 quality of bunt-resistance to their progeny. This was accordingly done with 

 different strains of AUora Spring, and the lowest percentage of bunty plants 

 was 87.1, and the highest 95.5. While carrying out these experiments, 

 Farrer hit upon the idea of selecting clean plants from the strains of his crosses 

 which showed the smallest percentage of bunt, in order to see if bunt-resis- 

 tance could be increased by a course of systematic selection. He observed 

 that the plants of the variable generation of a cross differed widely in their 

 liability to bunt, just as has been observed in the case of rust, and he came to 

 the conclusion that if the plants of this generation were ex]iosed to infection, 



